This invention relates to a method of and means for testing a glancing-incidence mirror system for an X-ray telescope, and more particularly, for a Wolter telescope.
Wolter telescopes, particularly type I telescopes, have been widely used for solar X-ray studies aboard space vehicles, and the results obtained to date suggest such telescopes hold great promise for future investigation of cosmic X-ray phenomena. A Wolter telescope uses glancing-incidence reflection optics based on internal conic sections of revolution about the optical axis of the telescope. To obtain good images, it has been demonstrated that an even number of reflecting surfaces are required. In a type I telescope, the optics are in the form of a glancing-incidence paraboloidal primary and a coaxial and confocal hyperboloidal secondary. That is to say, the primary is formed by a sleeve-like member whose interior surface is a portion of a paraboloid of revolution about the axis of the sleeve; and the secondary is formed by another sleeve-like member abutting the first member, and having an interior surface that is a portion of a hyperboloid of revolution about the sleeve axis. Thus, both conic sections have a common axis, and the sections are chosen so that they share a common focus.
It is well known that the percentage of an X-ray beam reflected from a surface is functionally related to the wavelength of the radiation and the angle at which the beam strikes the surface. In general, for a given angle, the longer the wavelength, the greater the percentage of X-rays reflected; and as the angle between the beam and the surface decreases, the percentage of the beam reflected increases. Suitable results are achieved when the angle is of the order of magnitude of 1.degree., hence the term glancing-incidence.
Para-axial X-rays in the annular region defined by the projection of a properly designed paraboloid on a plane perpendicular to the optical axis will strike the internal surface of the paraboloid mirror at less than the critical angle and, as a consequence, such rays will be almost totally reflected toward the focus of the paraboloid. Most of these reflected rays are then intercepted by the internal surface of the hyperboloid mirror. If this mirror is properly configured, the intercepted rays will again be almost totally reflected and converge at the paraboloid-hyperboloid focus. Because X-rays passing through this focus have been reflected from two surfaces, relatively good images will be obtained.
Early telescopes (ca 1963) utilized machined aluminum or cast epoxy mirrors, but the surface configuration and finish were such that the telescopes had reflecting efficiencies of about 1% and a resolution of about several arc minutes. Improved materials and fabrication techniques since then have significantly improved these parameters.
It is recognized that the angular resolution is relatively insensitive to local surface finish, and is limited, essentially, by the surface tolerances. Because imaging tests using visible light are not sufficiently sensitive to the surface defects in mirrors used at grazing incidence, special methods of testing have been employed. In one such method, the departure of the surface from a cone as a function of the distance along the axis thereof is determined by placing a glass test plate of known profile in contact with the reflecting surface and observing the pattern of interference fringes. In an actual telescope tested in this manner, it developed that the actual resolution achieved during use in flight was significantly better than ground based test results indicating that the resolution was limited by the laboratory test arrangement rather than by the telescope.
As to the effect of surface finish on efficiency, it is well known that the present state of the art of finishing mirrors produces irregularities that greatly exceed the Rayleigh criterion for a perfect reflecting surface. A direct technique for observing irregularities remains to be developed. However, new techniques promise to reduce deviations from the desired surface profile and reduce local surface rangliness.
There remains, however, the basic problem of testing a telescope on the ground before launch in a way that simulates the actual conditions of use. In other words, the telescope is to be used to record phenomena originating at galactic distances so that X-ray beams incident on the telescope are essentially parallel; but it has, heretofore, been very difficult to simulate this situation.
The difficulty arises because X-rays cannot conveniently be focused or made parallel by conventional optics, as can visible light. Therefore, the X-ray source is usually placed at a large distance from the device to be tested to approximate, to the desired degree, a source at infinity. Because X-rays are strongly absorbed by the constituents of the atmosphere at normal pressure, an evacuated path is required. This combination of long path and high vacuum is costly and results in a large, unwieldy machine that can test only a limited portion of the optics of a telescope.
While devices for collimating X-rays are known, these devices are complex and expensive, and usually absorb a substantial portion of the X-rays they are supposed to collimate.
It is therefore an object of the present invention to provide a new and improved method of and means for testing a glancing-incidence mirror system for an X-ray telescope wherein the above-described deficiencies are substantially overcome or reduced.